As the membrane filtration technique for separating and condensing a solvent, a solute or an insoluble substance in a solution there are known ones by reverse osmosis, pervaporation or ultrafiltration. Desalting of sea water with a reverse osmosis membrane having a mean pore size of, in general, at most 50 .ANG.(0.005 .mu.m) is already partly practiced. However, since this filtration with a reverse osmosis membrane is usually carried out under a high pressure of 20 to 50 atmospheres and the permeability coefficient(Pe) is very small, i.e., 10.sup.-14 (cm.sup.2 /sec.cmHg), the filtration efficiency is very poor and the apparatus employed must be disadvantageously enlarged. The mean pore size of a porous membrane employed in the pervaporation is generally as large as that in the reverse osmosis or not more than 100 .ANG.. According to this method one side of the porous membrane is kept in a vacuum and a solvent in a vaporous state is allowed to permeate through the porous membrane and is cooled and condensed. A number of studies have been made on the pervaporation as the method of separating and condensing a solvent in a solution. In this method the pressure difference(.DELTA.P) is one atmosphere and the separation coefficient(.alpha.) is at most about 25 under the present situation. Since the permeability coefficient(Pe) is verl low, i.e., 10.sup.-10 (cm.sup.2 /sec.cmHg) and a large amount of energy is necessary for maintaining the vacuum state and cooling the permeated solvent, it may be said that this technique is still far from practice.
An organic liquid in a mixture thereof in the state of one phase cannot be separated and condensed by the conventional operation conditions under pressure in the ultrafiltration using a porous membrane having a 2ra of at least 10.sup.-6 cm, which, accordingly, has not been taken into consideration for the separation and condensation of an organic liquid. Further, it has been considered that the separation and condensation of an organic liquid in a mixture thereof in the state of one phase using a porous membrane having a 2ra of not less than 10.sup.-6 cm is theoretically impossible.
As stated above, under the present situation there cannot be found a membrane filtration technique having a large Pe and a large .alpha. at the same time for the separation and condensation of an organic liquid in a mixture thereof in the state of one phase among the generally known membrane filtration techniques.
It is generally said that in order to increase the permeation rate(J) per unit area of a porous membrane, either the porosity(Pr) and the mean pore radius(ra) of the porous membrane and the pressure difference(.DELTA.P) are preferably increased or the thickness(d) is preferably thinned. However, in the conventional operation conditions under pressure or reduced pressure there are a negative correlation between the J and the .alpha. at .alpha..gtoreq.1 and a positive correlation between them at .alpha..ltoreq.1, and when the J is increased, the .alpha. approaches 1 without exception. Thus it has been considered impossible to carry out separation at a high efficiency with increased J and .alpha. at the same time in ultrafiltration.
On the other hand, as the method of separating and condensing a selected organic liquid in a mixture thereof in the state of one phase by utilizing the difference in solubility among the liquid components in the mixture there is known a method of separation by liquid-liquid extraction. More specifically, a specified organic liquid for extraction is mixed with the mixture in the state of one phase, and the resultant mixture is stirred to form a state of dispersion of fine particles consisting mainly of the mixture in the state of one phase or the liquid for extraction in which the selected organic liquid is dissolved. Then the dispersion thus formed is left to stand to separate the dispersion into two phases, and the upper and lower layers formed are collected, respectively. According to this method the standing of the dispersion is essential and has difficulty in continuously conducting the separation and recovery steps. Furthermore, when the standing of the dispersion requires a long period of time, a large apparatus for extraction and standing is necessary, and a large amount of the liquid for extraction remains in this procedures. If the liquid for extraction is expensive, the cost for the separation of the liquid disadvantageously becomes high. Especially when the difference in density between a disperse phase and a dispersion medium is small, it is impossible to separate the dispersion into two phases by standing and as a result, the method of separation by liquid-liquid extraction cannot be employed. Thus, with respect to the separation and condensation of ethanol from its aqueous solution there is no membrane filtration technique which can be practiced on an industrial scale.